Phase 6 of the integration plan in `.claude/knowledge/
simd-dispatch-architecture.md`.

Adds a new `tier4-avx512-check` job that compiles the crate with
`-Ctarget-cpu=x86-64-v4` so the `#[cfg(target_feature = "avx512f")]`
dispatch arm in `src/simd.rs` is exercised on every PR. Without this
the AVX-512 code path bit-rots under the v3 default (`x86-64-v3` baseline
in `.cargo/config.toml`) — it compiles only when a developer happens to
build locally with `--config .cargo/config-avx512.toml`.

Implementation notes
--------------------

* `cargo check` instead of `cargo test`/`cargo build`. GH-hosted
  `ubuntu-latest` runners have intermittent AVX-512 silicon across VM
  SKUs (Azure D-series mix); a v4-baked binary would SIGILL on a
  non-AVX-512 host. `check` compiles through type/borrow/monomorphization
  without producing a runnable artifact — catches the dispatch-arm
  type mismatches that motivated this PR series in the first place
  (PR #170 CI failure mode).

* Job-level `env: RUSTFLAGS: "-D warnings -Ctarget-cpu=x86-64-v4"`
  overrides the global `RUSTFLAGS="-D warnings"` set at the top of
  `ci.yaml`. Without the override, `.cargo/config-avx512.toml`'s
  rustflags would be ignored — env wins over config file in cargo's
  precedence (the same issue that broke PR #172 with the v3 setting
  in config.toml).

* Two check passes: default features + `hpc-extras`. The latter pulls
  the p64/fractal dep tree which exercises a different slice of the
  AVX-512 codepaths (BF16 RNE, AMX byte-asm). Each runs ~30 s with
  Swatinem cache hit, ~3 min cold.

* Added to the `conclusion` job's `needs` list so a v4 check failure
  blocks merge.

The previous iteration of tier4-avx512-check set `RUSTFLAGS=
"-Ctarget-cpu=x86-64-v4"` as a job-level env. That env applies to BOTH
the target compilation AND host build scripts (`build.rs` artifacts
cargo runs natively). On a GH-hosted runner without AVX-512 silicon,
those v4-baked build scripts SIGILL during dep compilation — the job
exited in 23 s before our own crate even started compiling.

Fix: use `CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS` (the env form
that's documented to apply only when cargo produces artifacts for that
triple, NOT to host build scripts) plus explicit `--target=x86_64-
unknown-linux-gnu` so cargo distinguishes host from target even when
they share the triple. Result: v4 reaches our crate, baseline reaches
build scripts.

Cargo doc reference: https://doc.rust-lang.org/cargo/reference/config.html
#target<triple>rustflags — "These flags only apply to the final
artifact, and won't affect dependencies."